1. Field of the Invention
The present invention relates to a process for producing an optically active 2-hydroxy-4-arylbutyric acid or its ester, an intermediate for synthesis of enalapryl or cilazapryl which is known as an excellent anti-hypertensive agent. The present invention further relates to a process for producing an optically active 2-hydroxy-4-phenylbutyric acid or its ester.
2. Description of the Prior Art
Known processes for producing an optically active 2-hydroxy-4-phenylbutyric acid include:
(i) a process in which racemic 2-hydroxy-4-phenylbutyric acid is used as a starting material and is subjected to an enzymatic reaction in the presence of an acylating agent to obtain the (R)-isomer (Agric. Biol. Chem., 55, 293, 1991), PA1 (ii) a process in which racemic 2-hydroxy-4-phenylbutyric acid is formed into a salt with an optically active amine, and this salt is resolved using a difference in solubility in a diastereomer (EP 329,156), PA1 (iii) a process in which 2-oxo-4-phenylbutyric acid is asymmetrically reduced through an enzymatic reaction J. Biotechnol., 24, 315 (1992)!, PA1 (iv) a process in which 2-oxo-4-phenylbutyric acid is asymmetrically hydrogenated Std. Surf. Sci. Catal., 78, 139 (1993)!, PA1 (v) a process in which D-homophenylalanine is reacted with nitrous acid to form (R)-2-hydroxy-4-phenylbutyric acid (U.S. Pat. No. 5,225,408), PA1 (vi) a process in which benzalpyruvic acid is formed into (R)-2-hydroxy-4-phenyl-3-butenic acid using an L-proline-sodium boron hydride composite, and this butenic acid is then catalytically hydrogenated to form (R)-2-hydroxy-4-phenylbutyric acid Japanese Laid-Open Patent Application (Kokai) No. 18,050/1992!, PA1 (vii) a process in which a racemic 2-hydroxy-4-phenylbutyric acid ester is subjected to an enzymatic reaction to hydrolyze the (S)-isomer alone and obtain an ester of the (R)-isomer Japanese Laid-Open Patent Application (Kokai) No. 225,499/1989!, PA1 (viii) a process in which a racemic 2-acyloxy-4-phenylbutyric acid ester is subjected to an enzymatic reaction to deacylate the (R)-isomer alone and obtain the (R)-hydroxy compound Japanese Laid-Open Patent Application (Kokai) No. 247,100/1989!, PA1 (ix) a process in which a 2-oxo-4-phenylbutyric acid ester is asymmetrically reduced through an enzymatic reaction Japanese Laid-Open Patent Application (Kokai) No. 328,984/1993!, PA1 (x) a process in which racemic 2-hydroxy-4-phenylbutyric acid is optically resolved and then esterified J. Chem. Soc., Perkin Trans. 1, 1011 (1986)!, PA1 (xi) a process in which a 1-menthol ester of 2-oxo-4-phenylbutyric acid is synthesized, and then reduced Tetrahedron Lett., 423 (1988)!, and PA1 (xii) a process in which a 2-oxo-4-phenylbutyric acid ester is asymmetrically hydrogenated (EP 206,993 and EP 564,406). PA1 which comprises reacting optically active acyloxysuccinic anhydride of formula (I): ##STR2## wherein R.sup.1 and * are as defined above, with a substituted or unsubstituted aromatic compound having from 4 to 10 carbon atoms in the presence of a Lewis acid as a catalyst. PA1 which comprises subjecting an optically active 2-acyloxy-4-oxo-4-arylbutyric acid of formula (II) to catalytic reduction of the carbonyl group in the 4-position, elimination of the acyl group, and esterification with a substituted or unsubstituted linear, branched or cyclic alkyl alcohol having from 1 to 6 carbon atoms or with an aralkyl alcohol having from 7 to 12 carbon atoms.
However, the processes (i), (ii), (vii), (viii) and (x) using the racemic 2-hydroxy-4-phenylbutyric acid ester as a starting material are problematic in that the yield does not exceed 50%. Further, in the processes (iii), (iv), (ix), (xi) and (xii) using 2-oxo-4-phenylbutyric acid or its ester as a starting material, a step of forming the 2-oxo-4-phenylbutyric acid ester is intricate, and the yield is not satisfactory. In the process (v) using D-homophenylalanine as a starting material, the amino acid is itself not a natural-type amino acid, and therefore its production cost is high. Thus, this process is not practical.
Still further, since the process (vi) using benzalpyruvic acid as a starting material comprises two steps, it is intricate and entails a high cost.